This invention relates to a drum for a rotary drum-type dehydrator. More specifically, this invention relates to a unique type of flighting in a drum-type dehydrator wherein pieces of material to be dried, such as wood chips, hog fuel and bagasse can be momentarily separated, turned and more evenly exposed to hot drying gases passed through the drum to more evenly dry all of the pieces while maintaining a steady rate of conveyance of the materials through the dehydrator.
Dryers of the type of this invention are commonly known in the pulp and paper industry as rotary wafer dryers. These dryers are used to dry large pieces of material such as wood chips, hog fuel or bagasse, which are dried before further processing into small pieces used, for example, to produce waferboard. Typically, wood chips dried by these dryers might range in size from about 0.02 to about 0.1 inch thick by 0.25 to about 3.0 inches wide by 2.75 to about 18 inches long wafers. The drum of these dryers is large, such as about 12 feet in diameter and about 20-40 feet in length. They are typically disposed to rotate about their longitudinal axis, which coextends with their center shaft in a horizontal plane or in a plane at a small angle to the horizontal.
Prior dehydrators of this type utilized a drum having a plurality of plate-like flights mounted to both the inner surface of the drum and the center shaft extending coaxially with the axis of drum rotation. These flights, which may be about 8-12 inches long, in the axially-extending direction, are arrayed in longitudinally-extending parallel rows. They extend radially outwardly from the shaft and have their ends bent. In operation, these flights carry the material to be dried as hot gases are introduced into one end of the drum and are removed from its other end. In the context of this invention, the term dehydrator is used to designate all parts of the apparatus, including framework, the source of supply of the hot gases, bearings and motor for rotatably supporting and driving the apparatus, and the dryer drum itself.
The purpose of flights in this type of dehydrator drum is to support most of the material passing through the drum in a plurality of relatively small groups of pieces which are spaced above the lowermost portion of the inner surface of the substantially horizontally disposed drum. In this way, the pieces of material intercept more of the hot drying gases passes through the dehydrator drum.
A problem with the operation of such prior dryers is that the plate-like flights tend to keep at least some of the pieces of material in approximately the same position on the flight, and location within the drum, for a longer period than is required to dry the pieces to the desired dryness. This causes plugging in the dryer as new pieces enter which, if not alleviated by removing dried pieces, can cause fires as well as produce large void areas downstream of the plugged section of the drum. Since no pieces of material are in the void areas to absorb the heat of the high temperature gases passing through the dryer, the gases exiting the dryer have a higher temperature than desired, thus lowering the thermal efficiency of the drying process. Further, the accumulated pieces tend to shield some pieces from the hot gases and prevent them from being dried to the desired dryness.
Some prior dryer designs stagger the successive flights of the plates along the length of the dryer drum, or shaft, to promote continuous movement of the pieces of material in the longitudinal direction through the dryer. However, this does not produce a steady, continuous flow of material through the dryer because the plate-like flights still function to essentially carry the material without turning it or providing a tumbling action.
Another problem with the plate-like flights is they become quite hot during operation of the dehydrator. When pieces of wood and bagasse reside on their relatively large surface for a period of time, they can become scorched, which diminishes the quality and value of the dried product.
The problems associated with the plugging and uneven drying of the pieces of material in prior rotary drum dryers are obviated by this invention. In this apparatus, the flights comprise a plurality of tines which extend radially inwardly from the wall of the drum and radially outwardly from the core shaft. The tines have ends which are either individually arrayed to describe a helical path in the longitudinal direction of the drum, or individual short sections of tines are so arrayed. The tines are equally spaced circumferentially about the surfaces of the drum and shaft. The ends of either individual tines, or small groups of contiguous tines in the same longitudinal row, are bent to promote movement of the pieces in different directions as they fall from the tines under the influence of gravity and rotation of the drum. Not all of the ends of the tines need be bent, and the ends of the tines which are bent need not be bent in the same direction, or angle. In fact, bending the tines in different directions, but generally forwardly in the direction of drum rotation, enhances the tumbling movement of the pieces of material both between tines in the same longitudinally-extending row as well as between tines in the circumferential direction.
The tines promote flow of the material circumferentially as well as in the downstream, or longitudinal, direction along the axis of the shaft of the drum. This facilitates its passage through the drum without causing a blockage. In the process, the continuous movement of the material around and between the tines promotes uniform drying.
Accordingly, it is an object of this invention to provide a drum for a dehydrator wherein the flights in the drum are comprised of tines.
Another object of this invention is to provide apparatus for a drum-type dehydrator which promotes uniform and efficient transfer of heat from hot gases to pieces of material passing through the drum.
Another object of the invention is to provide a rotary drum for a dehydrator which incorporates a flight arrangement which promotes uniform drying among the pieces of material being passed through the dehydrator.
Still another object of the invention is to provide a tine flight arrangement in a dehydrator drum which alleviates jamming, cording and wedging of wafers passing through the drum.
An advantage of the invention is the maximization of the exposure of material to flowing gases in a rotary drum dehydrator.
These and other objects, features and advantages of the invention will become more readily apparent to those skilled in the art upon reading the description of the preferred embodiment in conjunction with the attached drawings.